A mini-Explainer on the potency of secondhand marijuana smoke.

Recreational marijuana use became legal under Washington state law on Thursday and will likely be legal under Colorado law within a month. Jubilant smokers gathered under Seattle’s Space Needle to celebrate. Although smoking weed in public is still illegal, police looked the other way. Could a nonsmoking observer get high just from being near all that pot?

Only if he or she got very close. Researchers from the National Institute on Drug Abuse investigated the effects of “sidestream” marijuana—the smoke coming from the burning end of the joint—in 1986. They placed participants in an 8-by-7 room without ventilation and burned a series of marijuana cigarettes, removing the mainstream smoke through tubing. When the subjects were exposed to the smoke of four joints over the course of one hour, THC metabolites were detectable in some of their urine tests, but the tests were more often clean. The participants’ subjective assessment of “highness” was not significantly different from when they were exposed to placebo cigarettes.

Then the researchers intensified the experiment, burning 16 joints in an hour. After breathing the more highly concentrated smoke, the volunteers got indisputably high. Their subjective assessment of their mental states varied significantly from the placebo condition, and the urine tests consistently came back positive. The researchers noted that their results closely mirrored the effects of actively smoking one joint.

The take-home message from the study is that it’s possible, but difficult, to experience effects from sidestream pot smoke in normal social situations. The second phase of the protocol—burning 16 joints in a tiny room for an hour—is a scenario rarely experienced in ordinary life and certainly not comparable to being in a well-ventilated public place. Other studies in which participants have experienced legitimate contact highs have been equally extreme. In a Norwegian study, for example, volunteers were crammed in a car with six burning joints over a 30-minute period.